Selective etching has been considered for use as a technique in the manufacture of various semiconductor devices such as field-effect transistors (FET's). This technique involves incorporating an "etch stop" layer into the device in order to prevent unintended etching of underlying structures. Such unintended etching leads to variations in the thickness in the active layers of FET's, which in turn results in undesired conductance variations of FET active channel regions. As an example, variation in the uniformity of the etched active layers associated with the MESFET devices on a single wafer has limited production of MESFET-based integrated circuits formed on layered semiconductor structures grown using molecular beam epitaxy (MBE) processing techniques.
A number of methods have been developed for improving the uniformity of devices realized upon MBE-grown semiconductor structures. Included among these methods are selective "dry" (e.g., plasma) etching, selective "wet" etching using chemical solutions, as well as nonselective spray gate recess etching. Selective etch stop systems have so far proven the most effective in enhancing device uniformity. One figure of merit used to characterize selective etching techniques is known as "etch selectivity", which is defined as the ratio of the etch rate of the semiconductor layer overlying the etch stop layer to the etch rate of the etch stop layer. Although dry etchants enable high etch selectivity, dry etchants tend to increase the risk of damage to semiconductor devices relative to wet etchants. On the other hand, the lower etch selectivity of wet etchants has often required the utilization of relatively thick etch stop layers to prevent inadvertent etching of underlying portions of the device. Unfortunately, such thick etch stop layers often degrade device performance relative to otherwise identical devices fabricated in the absence of an etch stop layer. For example, the performance of GaAs-based MESFET devices realized using AlGaAs etch stop layers of the requisite thickness has tended to degrade due to the resultant increased source access resistance.